To cope with ever increasing network traffic, high-density optical multiplexing technologies have been researched such as coherent optical orthogonal frequency division multiplexing (CO-OFDM) and Nyquist wavelength division multiplexing (WDM). CO-OFDM is a technology that places multiple optical signals with fine frequency intervals on the frequency axis by using orthogonality between signals. Nyquist WDM is a method of high-density wavelength division multiplexing for optical signals where the band is limited to the symbol rate frequency. Both communication methods are highly efficient in terms of use of frequencies or wavelengths.
A method of transmitting a signal in which multiple subcarrier signals multiplexed or densely placed as a single signal is also called “super-channel transmission”. Collectively transmitting multiple subcarrier signals realizes flexible, large-capacity optical communication. A method has been proposed that controls in advance (pre-emphasizes) the output power level to the transmission line for each subcarrier, to reduce crosstalk between the subcarriers, when executing super-channel transmission (see, for example, Patent Document 1). Also, a method has been proposed that sets the amount of attenuation of a subcarrier signal in an edge part of the band of a super-channel signal to be smaller, and sets the amount of attenuation of a subcarrier signal in a center part of the band of the super-channel signal to be greater, to prevent degradation of optical transmission quality (see, for example, Patent Document 2).
Further, a technology has been known that detects the wavelength of an optical signal by using an optical channel monitor, and corrects a shift between a center wavelength of the optical signal input into a wavelength selective switch, and a center wavelength of a filter transmission band of the wavelength selective switch (see, for example, Patent Document 3).